In U.S. Pat. Nos. 4,964,890, 4,982,014, and 5,210,328, which are incorporated herein by reference, Freitag et al describe the manufacture of 1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane (hereinafter called "Bisphenol TMC" or "BPTMC") by the reaction of 3,3,5-trimethylcyclohexanone ("TMC" or dihydroisophorone) with phenol in the presence of an acid condensation catalyst and an organic sulfur compound as a co-catalyst, preferably an alkane thiol. However, no method of saving or recycling the co-catalyst is disclosed, either for the sake of economy or for environmental reasons.